The Importance of pH Balance of Skin and the Consequences of Its Deregulation.

As mentioned above, there are endogenous factors, such as age or skin type, which influence the skin's pH value. However, certain external factors can deregulate skin pH.

Soaps are a case in point. Soaps have a basic pH of around 9 - 10, which disrupts the natural acid pH of the hydrolipidic film, resulting in dry skin and the risk of infection. We therefore recommend cleansers with a pH between 4 and 7.

Note: the same applies to solid shampoos, which do not respect the hydrolipidic film on the scalp.

Unlike soaps, which have a basic pH that disturbs the skin's pH, cosmetics with an acidic pH between 3 and 4, such as products based on vitamin C or fruit acids, are beneficial for the skin because the enzymes responsible for producing ceramides (lipids forming the skin barrier) have an optimal acidic pH. In this way, they reinforce the skin barrier. But be careful! Some products, such as lemon juice, whose properties are touted on the Internet, have a pH that is too acidic (2.4) and are therefore irritating to the skin.

What's more, using cosmetics unsuited to your skin type can deregulate the skin's pH. For example, the pH of acne-prone skin is alkaline. In this case, using soap, which also has an alkaline pH, aggravates acne because the bacteria responsible for acne proliferate at a basic pH.

Whatever your skin type, choose 

cosmetics with a pH between 3 and 7.

As a reminder on the importance of pH, the skin's acid pH of 5.5 preserves the integrity and cohesion of the stratum corneum, and protects the skin from microbial infection. In fact, when skin pH rises, the enzymes responsible for ceramides production, which have an acidic pH optimum, are inactivated, compromising the structure and function of the stratum corneum.

Furthermore, while lipid-synthesizing enzymes decrease, other enzymes increase at acid pH: serine proteases. The latter lead to a reduction in corneodesmosomes, by degradation of desmoglein-1, whose role is to ensure the cohesion of stratum corneum elements.

When the pH balance of skin is deregulated, the skin is no longer resistant to external aggression and no longer retains its moisture. This is conducive to the development of dermatoses such as atopic dermatitis. Indeed, it has been shown that the higher the pH, the more intense the itching and the greater the dryness.

In addition to the fact that in atopic dermatitis the structure of the stratum corneum is altered, colonization by Staphylococcus aureus is a common feature of affected patients and is considered a major pathogenic factor in atopic dermatitis. Staphylococcus aureus adhesion to human keratinocytes increases with rising pH.

The growth of protective bacteria in the skin microbiota is optimal at acidic pH levels, whereas pathogenic bacteria thrive at neutral and basic pH levels (Staphylococcus aureus, Cutibacterium acnes). As a result, skin pH deregulation disrupts the skin's microbiota and prevents antimicrobial protection. This increases the risk of infection. Moreover, serine proteases, which are activated by a high pH, trigger the synthesis of cytokines that cause inflammation.